A new bifunctional biobased molecule, the carbonated methyl ester of ricinoleic acid maleate (CME‐RAM), useful for polyurethanes, was obtained from castor oil and CO2. This molecule bears carbonate and maleate functionalities that can participate, respectively, in aza‐Michael and aminolysis reactions. Maleate moieties are strong Michael acceptors and they were introduced into the fatty molecule using mild conditions. The synthetic steps include maleinization of castor oil followed by the simultaneous esterification and transesterification at 60°C. The subsequent stages were the epoxidation with acidic resins and the carbonation with CO2. The best results for epoxidation were obtained using Amberlite IR‐120 as catalyst at 48°C (84% conversion of double bonds). The carbonation of the oxirane ring with CO2 at 130°C, 6 MPa CO2 and 6 h reaction afforded 98% conversion of epoxides. It was demonstrated that aza‐Michael reaction of maleate groups was faster than aminolysis of carbonate groups, using hexylamine as a model molecule. CME‐RAM was oligomerized with ethylenediamine and used in the formulation of a conventional rigid polyurethane. Petrochemical polyol and isocyanate can be partially replaced by this bio‐based oligomer, producing a rigid material with a high percentage of green component and good mechanical properties.